Heat treatment process for controlling retained austenite content of carburized part

Abstract

The invention provides a heat treatment process for controlling the retained austenite content of a carburized part. The heat treatment process comprises the steps of high-temperature tempering after carburization, subzero treatment after quenching, low-temperature tempering, subzero treatment and low-temperature tempering. Through fine regulation and control of technological parameters of all the processes, the heat-treated material meets the requirement for high hardness, the extremely-low retained austenite content is controlled, and meanwhile the method has the advantages of being advanced in technology, low in cost, easy and convenient to operate and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of carburized parts, and in particular relates to a heat treatment process for controlling the content of residual austenite in carburized parts. Background Art

[0002] The roller guide sleeve in a marine diesel engine is an important carburized part, and its performance is directly related to the overall operating efficiency and reliability of the diesel engine. The material of this part is 12CrNi3A, and the precision requirement is quite high. During service, the shape and position tolerances of the dimensions are very high, otherwise there is a risk of failure. In order to meet the high performance requirements of the roller guide sleeve, a specific carburizing depth and surface hardness need to be achieved during the heat treatment process, while controlling the residual austenite content. Specifically, the carburizing depth after heat treatment is required to be 0.55-0.85mm, the surface hardness is 61-64HRC, and the residual austenite content is ≤3%. These requirements make the heat treatment process of the roller guide sleeve extremely complex and challenging.

[0003] There are already some carburizing and quenching methods for 12CrNi3A materials in the prior art. For example, CN109355615A discloses a 12CrNi3A carburizing and quenching process, which aims to improve the carburizing effect through multi-stage heating, heat preservation, quenching, ice-cooling and low-temperature tempering. However, the process mainly focuses on how to increase the carbon concentration of the carburized layer and promote the formation of carbides, and does not pay enough attention to the control of the residual austenite content, which cannot meet the strict requirement of the residual austenite content ≤ 3%.

[0004] Another method, as disclosed in CN114908242A, controls the film-like retained austenite content of heavy-duty carburized gears. Although this process can obtain a high-hardness working surface and a specific organizational structure, it is based on other materials, 20Cr2Ni4A, 20CrNiMoA or 17Cr2Ni2MoVNb, 18Cr2Ni4WA hot-rolled steel bars, and is not suitable for 12CrNi3A materials. This process also cannot meet the comprehensive requirements of 12CrNi3A roller guides for carburizing depth, surface hardness and retained austenite content.

[0005] For 12CrNi3A materials, the method of reducing the content of residual austenite during heat treatment generally includes high temperature tempering, quenching, deep cryogenic treatment and low temperature tempering after carburizing. Specifically: according to the conventional heat treatment process (such as 930℃ carburizing, 650℃ high temperature tempering, 820℃ quenching, -130℃ deep cryogenic treatment, 160℃ low temperature tempering), although the surface hardness requirements can be met, the content of residual austenite can never reach the strict requirement of less than 3%. Even if you try to reduce the quenching temperature and the cooling temperature, you can't effectively reduce the content of residual austenite.

[0006] In summary, the carburizing and quenching method in the prior art cannot meet the strict requirements of 12CrNi3A roller guide sleeve on carburizing depth, surface hardness and retained austenite content. Therefore, it is necessary to develop a new heat treatment process that can effectively control the retained austenite content while ensuring the surface hardness and carburizing depth of the roller guide sleeve, so as to meet the high performance requirements of the marine diesel engine for the roller guide sleeve. Summary of the invention

[0007] In view of the fact that the carburizing and quenching methods in the prior art cannot meet the strict requirements of the 12CrNi3A roller guide sleeve on the carburizing depth, surface hardness and residual austenite content, the present invention proposes an innovative heat treatment process. The process aims to ensure the surface hardness and carburizing depth of the roller guide sleeve while effectively controlling the residual austenite content to meet the high performance requirements of the roller guide sleeve for marine diesel engines. To achieve the above objectives, the present invention provides a heat treatment process for controlling the residual austenite content of carburized parts, which includes carburizing, high temperature tempering, quenching, and subsequent deep cryogenic treatment and low temperature tempering steps, and in particular adopts a cyclic treatment of two deep cryogenic treatments and low temperature tempering, which effectively reduces the residual austenite content of the steel and meets the requirements of the surface hardness and residual austenite content of the roller guide sleeve of the marine diesel engine.

[0008] The technical solution of the present invention is as follows:

[0009] The present invention provides a heat treatment process for controlling the content of retained austenite in a carburized part, comprising the following steps:

[0010] S1, carburizing: Place the 12CrNi3WA material parts in a carburizing furnace, and pass the carburizing medium to perform carburizing treatment until the carburizing depth reaches 0.55-0.85mm;

[0011] S2, high temperature tempering: heat the 12CrNi3WA material after carburizing in step S1 to 650±10℃, with a heating rate of ≤200℃ / h, keep warm for 120-180min, and cool to room temperature with nitrogen;

[0012] S3, quenching: heating the 12CrNi3WA material after high temperature tempering in step S2 to 650±10℃, with a heating rate of ≤200℃ / h, keeping the temperature for 20-40min, and then heating to 820±10℃, keeping the temperature for 60-90min, and then oil cooling to room temperature;

[0013] S4, first cryogenic treatment: cooling the 12CrNi3WA material after quenching in step S3 to -190±10°C, with a cooling rate of ≤200°C / h, keeping the temperature for 30-40 minutes, and then air cooling to room temperature;

[0014] S5, first low temperature tempering: heating the 12CrNi3WA material after deep cryogenic treatment in step S4 to 160±10°C, with a heating rate of ≤200°C / h, keeping the temperature for 150-210min, and then air cooling to room temperature;

[0015] S6, second cryogenic treatment: cool the 12CrNi3WA material after low-temperature tempering in step S5 to -190±10°C, with a cooling rate of ≤200°C / h, keep warm for 30-40 minutes, and then air-cool to room temperature;

[0016] S7, second low temperature tempering: heat the 12CrNi3WA material after deep cryogenic treatment in step S6 to 160±10°C, with a heating rate of ≤200°C / h, keep the temperature for 150-210min, and then air cool to room temperature.

[0017] Through the above 7 steps, the present invention realizes effective control of the residual austenite content of 12CrNi3A material after carburizing and quenching. In particular, through the cyclic treatment of two deep cryogenic treatments and low temperature tempering, the residual austenite content is significantly reduced, while ensuring the surface hardness and carburizing depth of the roller guide cylinder.

[0018] Preferably, in step S1, the carburizing medium is acetylene or propane, etc. The flow rate of the carburizing medium is adjusted according to the specific carburizing depth. The present invention does not limit the carburizing medium and the carburizing process, and the carburizing treatment can be performed according to the technology known in the art.

[0019] Preferably, in step S1, after the carburizing medium is introduced, the carburizing temperature condition of the carburizing furnace is as follows: the 12CrNi3WA material parts are heated to 650±10°C with the carburizing furnace, the heating rate is ≤200°C / h, and the parts are kept warm for 30±10min, then the temperature is continued to be raised to 930±10°C, and the parts are kept warm for 270-300min, and the parts are cooled and taken out of the furnace.

[0020] Preferably, in step S1, the temperature of the part is increased to 930±10°C at a heating rate of ≤200°C / h.

[0021] Preferably, in step S3, the temperature of the part is increased to 820±10°C at a heating rate of ≤200°C / h.

[0022] For high-strength alloy carburizing steels such as 12CrNi3A, due to the high content of alloying elements, quenching and tempering are used after carburizing, and a large amount of residual austenite will be formed in the surface structure, which reduces the surface hardness and fatigue strength. Therefore, it is necessary to try to reduce the content of residual austenite during heat treatment. The method generally includes high-temperature tempering after carburizing. The tempering temperature is generally 600-650 degrees. After a certain period of heat preservation, alloy carbides are precipitated. During subsequent quenching and heating, the carbides cannot be completely dissolved in the austenite, resulting in a decrease in the content of alloying elements and carbon in the austenite, which increases the MS point, thereby reducing the amount of residual austenite after quenching. Cryogenic treatment is performed immediately after quenching, followed by low-temperature tempering. Cryogenic treatment can allow austenite to continue to transform into martensite, the remaining austenite content will be greatly reduced, and the surface hardness will be significantly improved. Appropriately reducing the quenching temperature can also reduce the content of residual austenite, but it will also lead to a decrease in surface hardness. Simply using conventional carburizing, high temperature tempering, cryogenic treatment, and low temperature tempering can reduce the content of retained austenite to a certain extent, but it cannot meet the high requirement of retained austenite ≤3%. Traditional heat treatment generally involves quenching followed by ice-cold treatment and then low temperature tempering, which is called one cycle. After two hot and cold cycles, the carbide distribution of the present invention is more uniform. Two cycles can increase the hardness and reduce the content of retained austenite compared to one cycle, while three cycles will reduce the hardness. After carburizing and quenching of the roller guide sleeve 12CrNi3A, two cycles of hot and cold cycle treatment (using a deep cold temperature of -190) can obtain a more ideal organizational state, which can meet the designed high hardness and extremely low residual austenite content requirements.

[0023] Therefore, the present invention has been verified through a large number of experiments, and it is shown that in order to simultaneously obtain 12CrNi3WA materials with high hardness, high matrix strength, and low content of residual austenite, it is necessary to simultaneously meet the following requirements during heat treatment: (1) high temperature tempering after carburizing; (2) relatively low quenching temperature; (3) immediate ice-cooling treatment after quenching; (4) low temperature tempering after deep cryogenic treatment needs to be repeated twice. High temperature tempering after carburizing can reduce the content of residual austenite, and a relatively low quenching temperature can further reduce the content of residual austenite; deep cryogenic treatment is immediately performed after quenching, and low temperature tempering after deep cryogenic treatment needs to be repeated twice. While meeting the above conditions, it is also necessary to optimize and confirm the specific experimental parameters of each step. The relevant specific parameters are as described above. Any experimental parameters that exceed the above parameter value range will affect the final performance of the material, resulting in failure to meet the application index requirements of the material.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention performs high-temperature tempering after carburizing, appropriately reduces the quenching temperature of the steel, and immediately performs deep cryogenic treatment after quenching. Two cycles of low-temperature tempering after deep cryogenic treatment are required, which effectively reduces the residual austenite content of the 12CrNi3A material and meets the requirements of high surface hardness and low residual austenite content of the roller guide cylinder of a marine diesel engine.

[0026] (2) The present invention achieves comprehensive optimization of material properties by precisely controlling heat treatment process parameters while ensuring the surface hardness and carburizing depth of the 12CrNi3WA roller guide cylinder.

[0027] (3) The present invention solves the technical problem that the technicians in this field have long been eager to solve but have never succeeded in solving, that is, the 12CrNi3WA material meets the requirements of high hardness and extremely low residual austenite content. At the same time, the present invention also has the advantages of advanced technology, low cost and simple operation. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below in conjunction with specific embodiments to provide a better understanding of the content of the present invention, but the present invention is not limited to the following embodiments.

[0029] Embodiment 1:

[0030] Before loading into the carburizing furnace, ensure that the surface of the roller guide sleeve is free from defects such as rust and cracks, and the surface finish reaches Ra1.6. The chemical composition of 12CrNi3WA used for the roller guide sleeve is shown in Table 1.

[0031] Table 1 Chemical composition of 12CrNi3A for roller guide

[0032] Chemical Elements C Mn Cr Ni Si content(%) 0.15 0.40 0.78 2.99 0.28 Technical requirements 0.10～0.17 0.3～0.6 0.6-0.9 2.75-3.15 0.17～0.37

[0033] This embodiment provides a heat treatment process for carburizing parts to control the content of retained austenite, comprising the following steps:

[0034] S1, carburizing: Place the 12CrNi3WA material parts in a carburizing furnace, and introduce acetylene carburizing medium for carburizing treatment until the carburizing depth reaches 0.55-0.85mm;

[0035] The carburizing temperature condition of the carburizing furnace after the carburizing medium is introduced is: the 12CrNi3WA material parts are heated to 650°C (≤200°C / h) with the carburizing furnace, kept warm for 30±10min, then continued to heat to 930°C, kept warm for 270-300min, cooled down and then taken out of the furnace;

[0036] S2, high temperature tempering: put the 12CrNi3WA material obtained in step S1 into a holding furnace and heat it to 650°C (≤200°C / h) with the furnace, keep it warm for 120-180min, cool it to room temperature through a nitrogen furnace and take it out of the furnace;

[0037] S3, quenching: put the 12CrNi3WA material obtained in step S2 back into the holding furnace and heat it to 650°C (≤200°C / h) with the furnace, keep it warm for 20-40 minutes, continue to heat it to 820°C, keep it warm for 60-90 minutes, and then cool it to room temperature with oil;

[0038] S4, first cryogenic treatment: put the 12CrNi3WA material obtained in step S3 back into the holding furnace and cool it down to -190°C (≤200°C / h) with the furnace, keep it warm for 30-40 minutes, then take it out of the furnace and air cool it to room temperature.

[0039] S5, first tempering: put the 12CrNi3WA material obtained in step S4 back into the holding furnace and heat it to 160°C (≤200°C / h) for 150-210 minutes. After the heat preservation, take it out of the furnace and air cool it to room temperature.

[0040] S6, second cryogenic treatment: put the 12CrNi3WA material obtained in step S5 back into the holding furnace and cool it down to -190°C (≤200°C / h) with the furnace, keep it warm for 30-40 minutes, then take it out of the furnace and air cool it to room temperature.

[0041] Step S7, second tempering: the 12CrNi3WA material obtained in step S6 is put back into the holding furnace and heated to 160°C (≤200°C / h) with the furnace, and kept warm for 150-210 minutes. After the heat preservation, it is taken out of the furnace and air-cooled to room temperature.

[0042] After completing the above heat treatment process, the surface hardness and residual austenite content of the roller guide cylinder are tested.

[0043] Embodiment 2~3:

[0044] The steps of Examples 2 to 3 are the same as those of Example 1, but the relevant parameters in the steps are different, as shown in Table 2.

[0045] Comparative Examples 1-7:

[0046] The steps of Comparative Examples 1 to 7 are the same as those of Example 1, but Comparative Example 1 only uses one cycle; Comparative Example 2 uses three cycles; the cooling temperature of Comparative Example 3 is -130°C, and two cycles are used; the cooling temperature of Comparative Example 4 is -130°C, and one cycle is used; the cooling temperature of Comparative Example 5 is -130°C, and three cycles are used; the quenching temperature of Comparative Example 6 is 840°C, and two cycles are used; the quenching temperature of Comparative Example 7 is 800°C, and two cycles are used; as shown in Table 2.

[0047] Table 2 Experimental parameters of each step of the embodiment and comparative example

[0048]

[0049]

[0050] Table 3 shows the design requirements of the 12CrNi3WA material for the roller guide sleeve, and Table 4 shows the mechanical properties of the material obtained after the heat treatment process described in the present invention.

[0051] Table 3 Design requirements for 12CrNi3WA material for roller guide

[0052]

[0053] Table 4 Experimental data obtained after using the heat treatment process of the present invention

[0054]

[0055]

[0056] It can be seen from the test results of the embodiments and comparative examples that it is difficult to achieve high hardness and low residual austenite content for 12CrNi3WA material at the same time. After the main residual austenite content is reduced to a certain amount, simply increasing or decreasing individual parameters cannot meet the design requirements. Therefore, as mentioned above, in order to simultaneously obtain 12CrNi3WA material with high hardness and low residual austenite content, it is necessary to simultaneously meet the following conditions during heat treatment: (1) high temperature tempering after carburizing; (2) relatively low quenching temperature; (3) ice cold treatment immediately after quenching; (4) low temperature tempering after deep cold treatment requires two cycles. While meeting the above conditions, it is also necessary to optimize and confirm the specific experimental parameters of each step. The relevant specific parameters are as described above. Any experimental parameters that exceed the above parameter value range will affect the final performance of the material, resulting in the failure to meet the application index requirements of the material. Using the heat treatment process provided by the present invention, the surface hardness and residual austenite content of the obtained roller guide tube 12CrNi3WA material meet the design requirements. Furthermore, the present invention has the advantages of advanced technology, low cost and simple operation, which not only meets the requirements of high hardness, but also achieves the requirements of extremely low residual austenite content.

[0057] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A heat treatment process for controlling the content of retained austenite in carburized parts, characterized in that: The following steps are involved: S1, carburizing: Place the 12CrNi3WA material parts in a carburizing furnace, and pass the carburizing medium to perform carburizing treatment until the carburizing depth reaches 0.55-0.85mm; S2, high temperature tempering: heat the 12CrNi3WA material after carburizing in step S1 to 650±10℃, with a heating rate of ≤200℃ / h, keep warm for 120-180min, and cool to room temperature with nitrogen; S3, quenching: heating the 12CrNi3WA material after high temperature tempering in step S2 to 650±10℃, with a heating rate of ≤200℃ / h, keeping the temperature for 20-40min, and then heating to 820±10℃, keeping the temperature for 60-90min, and then oil cooling to room temperature; S4, first cryogenic treatment: cooling the 12CrNi3WA material after quenching in step S3 to -190±10°C, with a cooling rate of ≤200°C / h, keeping the temperature for 30-40 minutes, and then air cooling to room temperature; S5, first low temperature tempering: heating the 12CrNi3WA material after deep cryogenic treatment in step S4 to 160±10°C, with a heating rate of ≤200°C / h, keeping the temperature for 150-210min, and then air cooling to room temperature; S6, second cryogenic treatment: cool the 12CrNi3WA material after low-temperature tempering in step S5 to -190±10°C, with a cooling rate of ≤200°C / h, keep warm for 30-40 minutes, and then air-cool to room temperature; S7, second low temperature tempering: heat the 12CrNi3WA material after deep cryogenic treatment in step S6 to 160±10°C, with a heating rate of ≤200°C / h, keep the temperature for 150-210min, and then air cool to room temperature.

2. The heat treatment process according to claim 1, characterized in that: In step S1, the carburizing medium is acetylene or propane.

3. The heat treatment process according to claim 1, characterized in that: In step S1, after the carburizing medium is introduced, the carburizing temperature condition of the carburizing furnace is as follows: the 12CrNi3WA material parts are heated to 650±10°C with the carburizing furnace, the heating rate is ≤200°C / h, and the temperature is kept for 30±10min, and then the temperature is continued to be raised to 930±10°C, and the temperature is kept for 270-300min, and the parts are cooled and taken out of the furnace.

4. The heat treatment process according to claim 3, characterized in that: In step S1, the part is heated to 930±10°C at a heating rate of ≤200°C / h.

5. The heat treatment process according to claim 1, characterized in that: In step S3, the temperature of the part is increased to 820±10°C at a heating rate of ≤200°C / h.

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